Air bags for protection of occupants of vehicles require that a substantial volume of gas quickly be delivered under pressure. There are numerous design constraints, among them being the small spaces available for storage of the inflator, often in the hub of a steering wheel, or in a door panel. In order to generate a sufficient volume of gas from a suitably small source, solid propellant materials are frequently utilized.
Such materials, while capable of generating the necessary volume of gas, involve many problems of their own. In manufacture they are hazardous, and the device must be assembled in conditions which arise to the standards of toxic propellant loading. After installation, these materials remain as an unseen risk to dismantlers, because they are hazardous materials. In the event of a vehicle fire, there is the risk of an unexpected the event of a vehicle fire, there is the risk of an unexpected explosion. Furthermore, they often generate undesirable gaseous products. Other constraints are that the output gases should not be in a condition to harm the bag itself. Compliance with these constraints is often attained at the cost of undesirable complexity.
It is an object of this invention to provide a gas source which consists only of benign "green" reactants, which if they leak are harmless to the environment or to nearby persons, and which will not autoignite. The gases produced by them are benign.
It is another object of this invention to provide a gas source whose output is not harmful to the bag itself, such as from particulates or from excessively hot inflation gases.
Because the air bag must be fully inflated within a few milliseconds, it could be presumed that a very steep rise in pressure is desirable. Such is not the actual situation. An air bag is multipli-folded and very tightly packed in order to be stored in a small volume. Too sudden a burst of gas might simply shoot the folded bag at the passenger. What is required is a rather slow initial pressure rise to allow the bag to begin to unfold, followed by a more rapid rise in pressure to inflate the bag quickly. The total event will still take only a few milliseconds, but there is assurance that the bag will unfold correctly.
As the art of air bags has developed, it has become increasingly evident that different inflation rates to different inflated pressures are desirable. For example, a small child who is belted in requires a less stressful restraint than a heavy passenger who is not belted in. The rate of increase of pressure to a desired maximum pressure, and the maximum pressure itself can advantageously be made very different.
Such variability cannot be attained with the use of solid propellant martials. Such materials are one shot, and develop gases according to their own stoichiometry. It is possible to throttle or by-pass some of the generated gases, but these techniques require specialized controls. Because air bags are generally designed for a 20 year installation life, any complexity is undesirable.
The use of a liquid propellant to drive the inflator offers, in addition to the advantages already discussed, means to vary the output rates and pressures which can be as simple as providing a plurality of igniters, which can be initiated (if at all) at different times, a plurality of exit orifice configurations which will be selectively opened, and means to vary the rate of flow of fuel to the oxidizer. All of these are readily established by sensors which are responsive, for example, to whether a seat is occupied, whether the occupant is belted, and how much the occupant weighs.
Accordingly, an air bag need not be deployed into an empty, seat, and a child need not face the same vigorous restraint as a heavy adult.
It is therefore an object of this invention to provide an air bag inflator which utilizes "green" constituents, and enables all or any of the above additional advantages to be attained.